Crosslinked polyethylene is polyethylene modified such that excellent insulation property is maintained and thermally unstable defect is improved, and it has a three-dimensional network structure unlike linear polyethylene. The crosslinked polyethylene is known to have excellent heat resistance, durability, chemical resistance, flexibility, etc., and due to these properties, it is being widely used for electrical insulation of a power cable.
In general, a power cable is prepared by mixing polyethylene with a crosslinking agent and an antioxidant, etc., and extrusion molding. During the extrusion molding process, crosslinking of polyethylene is progressed. And, in addition to the crosslinking agent and antioxidant, other components, for example, a crosslinking accelerator, treeing inhibitor, etc. are used together, as necessary.
However, when preparing the power cable, the migration of additives occurs, which causes deviation of extrusion amount. The migration of additives means a phenomenon wherein additive components having relatively low molecular weight compared to polyethylene move to the surface during extrusion or after extrusion. Thus, extruded outer diameter deviation of a power cable is generated to cause product defect.
In order to minimize the migration of additives, previously, focus was on controlling of the kind of additives and added amount. Although this has the effect of reducing the migration of additives, the use of additives required according to the properties required in a power cable is limited, and thus, it cannot be a fundamental solution.
Therefore, the present inventors thoroughly examined the additive migration phenomenon, and as the result, confirmed that the oil content of polyethylene used for the preparation of crosslinked polyethylene is closely related to the migration of additives, and that by controlling the oil content, the migration of additives is inhibited, and completed the present invention.